I consulted the 'Official Notes' and they said that we should not claim to know the intention of others and condemn them out of ignorance.
I have tattoos, and sometimes I am asked about them, mostly out of curiosity and not condemnation. I wonder what life would be like if people with tattoos ran around asking everyone who does not have one to justify their choice. LOL
Cultural norms obviously also matter - in some places, not having a tattoo or piercing would be considered odd. And it is the way in nature that the 'ab-normal' are rejected by the rest of the pack/tribe/regiment etc.
This topic comes around every now and again, and in my experience many people have tattoos to mark events in their life to which they attach importance, not as something decorative. I know many Buddhists who have done so.
If we want to question why one person chooses a tattoo to mark, let's say, Taking Refuge, then do we also question the other outward indications of religious life which may be a shaved head, for example, or uncut hair, or wearing a kirpan? Is it any of our business?
If one person chooses to 'carry' a bell and dorje as a necklace, and another chooses a tattoo, rather than attack the person for their choice of medium, I would think how wonderful that a person chooses to keep those symbols with them at all times.
We don't start as perfect beings with pure minds, and don't all think of our bodies as impermanent bags of puss and bile. I can find nothing disrespectful or 'wrong action' if there is positive motivation and it helps that individual with their path at a particular time in their lives.
Here are some examples of Buddhist-themed Tattoos: http://tibetantattoos.tribe.net/photos